A radiation image recording and reproducing method utilizing a stimulable phosphor is described, for example, in U.S. Pat. No. 4,239,968. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release radiation energy stored in the phosphor in the form of light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. Thereafter, the radiation energy remaining in the radiation image storage panel may be removed from the panel, and the panel is stored for the next radiographic process.
In the radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to an object at a small dose. Therefore, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described method comprises a stimulable phosphor layer which may be provided on an appropriate support. Further, a transparent film may be provided on its free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The stimulable phosphor layer comprises a binder and stimulable phosphor particles dispersed in the binder. The stimulable phosphor emits light (i.e., gives stimulated emission) when excited with stimulating rays after having been exposed to a radiation such as X-rays.
The radiation image recording and reproducing method is generally performed in a united radiation image recording and reading apparatus which comprises recording means (for applying a radiation having an image information to the radiation image storage panel to record the radiation image on the storage panel); reading means (for irradiating the stimulating rays to the storage panel having the radiation image to produce stimulated emission from the storage panel and photoelectrically reading the stimulated emission); erasing means (for applying erasing light to the storage panel after the reading step is complete to remove a radiation image remaining in the storage panel); and transfer system (which is arranged between these means, for transferring the storage panel from one means to another means in predetermined order). Alternatively, the radiation image recording and reading apparatus may comprise two separated apparatuses, that is, a radiation image recording apparatus and a radiation image reading apparatus equipped with erasing means.
In any of the radiation image recording and reproducing systems, the radiation image storage panel is repeatedly employed after the remaining radiation image is erased. Particularly, in the former system using the united apparatuses, the radiation image storage panel can be employed repeatedly with successively transferring from one means to another means.
In the radiation image recording and reproducing method, the radiation image recorded in the storage panel is generally read by applying the stimulating rays to one side of the storage panel and collecting a light emitted by the phosphor particles by means of a light-collecting means from the same side (hereinafter referred to as "single-side reading system"). There is a case, however, that the light emitted by the phosphor particles should be collected on both sides of the storage panel. This is because the emitted light is desirably collected as much as possible. There also is a case that the radiation image recorded in the phosphor layer varies along the depth direction of the layer and such variation of the radiation image should be detected. An example of the system for reading radiation image from both sides (hereinafter referred to as "double-side reading system") is illustrated in FIG. 1 of the attached drawings.
In FIG. 1, the radiation image storage panel 11 is transferred (or moved) by a combination of two sets of nip rolls 12a, 12b. The stimulating rays such as laser beam 13 is applied to the storage panel 11 on one side, and the light emitted by the phosphor advances upward and downward (in other words, toward both the upper and lower surfaces). The downward light 14a is collected by a light collector 15a (arranged on the lower side), converted into an electric signal in a photoelectric conversion device (e.g., photomultiplier) 16a, multiplied in a multiplier 17a, and then sent to a signal processor 18. On the other hand, the upward light 14b is directly, or after reflection on a mirror 19, collected by a light collector 15b (arranged on the upper side), converted into an electric signal in a photoelectric conversion device (e.g., photomultiplier) 16b, multiplied in a multiplier 17b, and then sent to the signal processor 18. In the signal processor 18, the electric signals sent from the multipliers 17a, 17b are processed in a predetermined manner such as addition processing or reduction processing depending on the characteristic of the desired radiation image.
In the radiation image recording and reproducing method using the radiation image storage panel, the reproduced radiation image should have high sharpness and good graininess as is in the conventional radiation image reproducing method using a silver halide radiographic film. In order to improve the sharpness and graininess of a radiation image reproduced in the conventional radiation image recording and reproducing method in which the recorded radiation image is read from one side of the radiation image storage panel, i.e., single-side reading system, it has been proposed to color the stimulable phosphor layer with a colorant (i.e., coloring material) capable of absorbing the stimulating rays or to vary the weight ratio of the phosphor particles to the binder in the phosphor layer along the depth direction so that the ratio can become higher on one side. Some of such proposals have been practically adopted.
For the radiation image recording and reproducing method in which the recorded radiation image is read by both sides, i.e., double-side reading system, however, there have been given no proposals to improve the graininess and sharpness of the radiation image reproduced in the method. The improvement of the graininess and sharpness of the radiation image reproduced in the double-side reading system should be achieved by modification of the radiation image storage panel in a way differing from that for the single-side reading system.